1. The Field of the Invention
The present invention relates to semiconductor lasers. More particularly, the present invention relates to a vertical cavity surface emitting laser that is configured to operate in a wide temperature range.
2. Background and Relevant Art
One of the light sources used in optical communication systems is a vertical cavity surface emitting laser (VCSEL). VCSELs are popular in part because they can be manufactured in large quantities due to their relatively small size and can be tested in wafer form. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well to optical fibers.
In a VCSEL, the light or optical signal being amplified resonates in a direction that is perpendicular to the pn-junction. The cavity or active region of a VCSEL is thus relatively short and a photon has a small chance of stimulating the emission of an additional photon with a single pass through the active region. To increase the likelihood of stimulating the emission of photons, VCSELs require highly efficient mirror systems such that a photon can make multiple passes through the active region. The reflectivity requirement of VCSELs cannot easily be achieved with metallic mirrors.
VCSELs thus employ Distributed Bragg Reflector (DBR) layers as mirrors. The semiconductor materials or dielectric materials used in DBR layers are grown or formed such that each layer has a refractive index that is different from the refractive index of adjoining layers. The junctions between the DBR layers that are grown in this fashion cause light to be reflected. The amount of light reflected, however, by a single junction is relatively small and is often dependent on the variance between the relative refractive indices of the adjoining materials. For this reason, a relatively large number of DBR layers are formed in a VCSEL in order to achieve high reflectivity. VCSELs, for example, often have on the order of 50 to 100 DBR layers in order to achieve sufficient reflectivity.
Even though forming a large number of DBR layers can be a difficult task, VCSELs are attractive for various low power and/or multi-transverse mode applications because they are easy to test, have a good beam profile and typically have low power consumption. These attributes also make VCSELs unattractive for other applications, such as those that require more power in a single mode. For example, high power pump lasers are required for erbium doped fiber amplifiers. The power supplied by the pump lasers should be delivered in a single mode in order to avoid excessive noise. Current VCSELs are not suitable for this application because single mode VCSELs do not produce sufficient power. Those VCSELs that do produce sufficient power introduce excessive noise because more than one mode is generated. As a result, higher cost edge emitting lasers are often used for these applications.
VCSELs are also limited to a relatively narrow operational temperature range meaning that VCSELs cannot be used in environments that experience wide ranges of temperatures. VCSELs are often said to be tuned to a particular temperature at which the threshold current required to operate the laser is minimized. The further the temperature is away from the particular tuned temperature, the more current is required to operate the laser. After the temperature drifts a certain amount away from the tuned temperature it is no longer possible to drive the laser because the current requirement is so high. In addition to requiring excessive current, the lifetime of the laser is also shortened in a logarithmic manner when the VCSEL is operated away from the particular tuned temperature. The following equation illustrates the mathematical relationship between the drive current of the VCSEL and the mean lifetime of the VCSEL: MTTFαI−ne(−fa/KTj). MTTF=Mean time to failure, I=Drive current, fa=activation energy, Tj=Junction temperature. As illustrated in the equation, it is undesirable to operate a VCSEL at a high drive current or threshold current because of the significant negative affect it has on the lifetime of the VCSEL. Because of this problem, a VCSEL can only be efficiently used in an environment where the temperature is controlled within a narrow range.
Therefore there is a need in the industry for a VCSEL module that is configured to operate in a wide temperature range without severely minimizing it's lifetime or requiring excessive drive current. The module should be easy to manufacture and not degrade the signal quality produced by the VCSEL in any manner.